1. Field of the Invention
The present invention relates generally to a transmission which is used on a vehicle, and more particularly to the configuration of a housing (i.e., a transmission case) used for a belt-type continuously variable transmission which comprises a pair of pulleys and a belt disposed around these pulleys.
2. Description of the Related Art
Power transmissions which transmit power by a belt disposed around a pair of pulleys have been well-known and are widely used. As such a transmission, a belt-type continuously variable transmission comprises a pair of pulleys, each pulley having a variable groove width, and a belt disposed therearound. The V-belt pitch radii of the pulleys are variably changed by adjusting the groove widths of the pulleys, so that the speed ratio of the transmission is continuously varied. It has been known that this type of transmission can be used on a vehicle as disclosed in Japanese Laid-Open Patent Publication No. H4(1992)-258528(A), which was filed by the present invention.
This vehicular continuously variable transmission comprises drive and driven pulleys, a belt, a forward-reverse selector mechanism, a starting mechanism, a reduction train, a differential mechanism, etc. in a hosing, and for lubricating these mechanisms, forced-feed lubrication or sprinkled lubrication by gear rotation is carried out. The oil used for this lubrication is also used as a hydraulic pressure controlling fluid (hydraulic oil) for controlling the adjustment of the groove widths of the drive and driven pulleys and for controlling a clutch.
The hydraulic pressure which is used for the forced-feed lubrication and for the hydraulic control is generated by a hydraulic pump which is driven by an engine through a gear or a chain mounted on a shaft that is coupled to the engine, and the hydraulic pressure is delivered to the components which require lubrication and to the components which require hydraulic pressure control. After the hydraulic pressure is used for the respective purposes, the hydraulic oil flows downward from the respective components. Therefore, the transmission includes an oil well or an oil pan to collect the hydraulic oil, which is then recirculated by the hydraulic pump.
The oil which has been used for the lubrication and for the actuation returns to the oil pan which is provided in the lower portion of the housing. However, in the room that accommodates the transmission mechanism which includes the above mentioned drive and driven pulleys (this room is hereinafter referred to as "the pulley chamber"), some oil is thrown from the pulleys radially outward because of the rotation of the pulleys (centrifugal force). In a similar manner, some oil is thrown from the belt approximately in the directions of the straight portions of the belt, near and at each inflection point where the direction of the motion of the belt drastically changes from a straight line, which is formed by the belt extending between the pulleys, to an arc, which is formed by the belt disposed around a respective pulley, because of the motion of the belt. The oil sprayed in this manner hits the inner walls of the pulley chamber and flows down along the inner walls into the oil pan.
More specifically, the oil flowing down along the inner walls of the pulley chamber is led into a inlet of an oil passage which is connected to the oil pan. When an large amount of lubrication oil is being used, for example, while the engine rotates at a high rotational speed, the amount of the oil flowing down along the inner wall of the chamber also increases, and the oil accumulates at the lower portion of the pulley chamber. In this situation, the friction from the lubrication oil which acts as a drag (rotational resistance) especially against the rotation of one of the drive and driven pulleys positioned lower in the chamber increases as the amount of the oil flowing down and accumulating in the chamber increases.
In such a belt-type continuously variable transmission, typically, to detect the rotational speed of the respective pulley, each of the drive and driven pulleys includes about 20 to 30 raised portions, and proximity sensors which sense these protrusions are provided in the housing. In a condition where the amount of oil used for lubrication increases as described above and where the oil accumulates at the bottom of the pulley chamber, the lubrication oil may be agitated by these raised portions, resulting in a problem of foaming of the lubrication oil. Also, the agitation of the oil at a high temperature, if it happens, promotes the oxidation and degradation of the lubrication oil.
To solve this problem, for example, Japanese Laid-Open Patent Publication No. H8(1996)-285023(A) discloses a method that provides a relatively large opening which leads directly to the oil pan, at the lower portion of the pulley chamber or an inlet for a discharge oil passage to return the oil which flows down along the inner walls to the oil pan, near the middle point of a line drawn between the two pulleys tangentially to the peripheries of the pulleys.
However, for example, the provision of a relatively large opening leading to the oil pan at the lower portion of the pulley chamber requires a compromise of the strength of the entire transmission housing including the pulley chamber, so there is a limit to the size of the opening. In addition, because the level of the oil in the oil pan varies in correspondence to the driving condition of the vehicle, one of the pulleys positioned lower than the other may happen to be immersed into the oil. In this case, the above mentioned friction increases further.
On the other hand, for the provision of an inlet opening for a discharge oil passage, near the middle point of a line tangent to the peripheries of the pulleys, a little effect has been confirmed in returning to the oil pan the oil which flows down along the inner wall on which the inlet opening is provided. However, some oil flows down along other walls, and some other oil drips directly from one of the pulleys positioned above the other. As a result, a certain amount of lubrication oil accumulates at the bottom of the pulley chamber, so a problem of agitation of the lubrication oil may occur as mentioned above.